There are many situations in which remote monitoring of various conditions is desirable. For example, up-to-the minute status of high voltage feeder circuit breakers in electrical substations have been monitored in a remote manner, as well as other state conditions sensors, such as pressure, temperature, liquid level and the like, intrusion sensors are often monitored in a remote manner as are smoke detectors. Recently, medical information and even the location of a person have been monitored in a remote manner.
In the past, such remote monitoring equipment has used land line communication between the monitoring equipment and the monitoring station. However, such land line communication links can be subject to several drawbacks. For example, in severe weather conditions or in wet climates, the land lines may be damaged by weather conditions or water. Such damage may degrade the link to such an extent that data transmitted over such link, if transmission is not totally interrupted, may become unreliable. Furthermore, land line communications may be subject to being deliberately interrupted by an intruder.
To overcome such problems, there have been several proposals in the art for including a radio link as a backup to a land line link. Such systems use the land line link if it is available, and switch to the radio link if the land line link is not operative.
While somewhat successful in avoiding the problems associated with a link consisting only of a land line, such land line/radio link systems still have certain drawbacks. For example, a radio link may not permit a system integrity check and there may not be a good method of determining the integrity of the link and ensuring that the monitoring station is on line with the subscriber premises station at all times. If, for example, the antenna at the subscriber station has been damaged, the link may not be properly established, and radio links may not be able to monitor such condition. Many radio link repeater site stations may be open and accessible to many people and thus may be subject to tampering.
Still another problem with using radio links is that several repeater stations may be required in addition to a base station to cover a prescribed area. Additionally, the radio transmitters at the subscriber premises may be restricted to low power outputs and may be able to communicate only within a certain geographic area and report to only one receiver site having no redundancy. Furthermore, if a repeater station fails, incoming signals may be lost.
Still further, radio transmitters may be restricted in the number of radio channels that can be used in a particular geographic area. This may require subscribers to share channels, and create a potentially undesirable situation. Furthermore, if two or more radio transmitters key at the same time, it may be possible that no communication is received at the monitoring station. System overload is also possible with such systems.
Yet a further problem with radio links is the limited adaptability thereof. For example, radio links may not be amenable to use with premises monitoring equipment that includes the transmission of slow scan, freeze frame, or real time CCTV, or to use with supervisory monitoring equipment that might include computer controlled fault-monitoring equipment. More importantly, such systems may not be completely usable with a system that includes downloading of information from the monitoring station to the subscriber station. That is, it may not be possible to efficiently send all of the information and data signals that are necessary for efficiently monitoring a premises from the monitoring station to the subscriber station over such links.
Yet a further problem with such systems is inherent in the switching system itself. That is, as mentioned above, these systems attempt to communicate with the monitoring station via a land line, and if such land line is inoperative, will switch to a radio link. This requirement may make the integrity of the system dependent on the switching system. Should this system become damaged or inoperative, the entire system will become inoperative. It can even be possible to view the overall integrity of the overall system as being a function of the integrity of the switching system. The switching operation may, in some cases, interrupt the data signal and introduce a spike into the signal, and therefore, influence the content and reliability of such signal. In some circumstances, the data can be of such a character that the switching operation may affect such data.
Therefore, systems that use only a land line link between a subscriber station and a monitoring station have problems, and those systems that include a radio link backup and switch to that radio link if the land line link is inoperative also have several restrictions and drawbacks.